1. Field of the Invention
The present invention relates to a heat exchanger for a refrigerating system where carbon dioxide (CO.sub.2), as a refrigerant, is used in a super-critical region of a refrigerating cycle.
2. Description of Related Art
Recently, it has been required to avoid the use of freon as a refrigerant in refrigerating systems. For example, JP-B-7-18602 discloses a vapor compression type refrigerating cycle (CO.sub.2 -refrigeranting cycle) where carbon dioxide (CO.sub.2) is used as a refrigerant in place of freon.
The CO.sub.2 -refrigeranting cycle operates in the same manner as the conventional vapor compression type refrigerating cycle does where the freon is used as a refrigerant. That is, as denoted by A-B-C-D-A in FIG. 7 (Mollier chart of the CO.sub.2 -refrigerating cycle), gas-phase CO.sub.2 is compressed (A-B) by a compressor to high-temperature and high-pressure super-critical phase CO.sub.2, and the super-critical phase CO.sub.2 is cooled (B-C) by a heat emitter (gas cooler). The super-critical phase CO.sub.2 is pressure-reduced (C-D) by a pressure reducer to a gas-liquid phase CO.sub.2, and the gas-liquid phase CO.sub.2 is evaporated (D-A) by an evaporator while cooling an outside fluid by absorbing heat from the outside fluid.
The CO.sub.2 changes from super-critical phase to gas-liquid phase when the pressure thereof becomes lover than a saturated liquid pressure (pressure at a cross point between a segment CD and a saturated liquid line in FIG. 7). When the CO.sub.2 changes from a condition (C) to a condition (D) slowly, the CO.sub.2 changes from the super-critical phase to the gas-liquid phase via liquid phase.
In the super-critical region, the molecule of CO.sub.2 moves as in the gas phase while the density of CO.sub.2 is substantially the same as the liquid-density thereof.
The critical temperature of CO.sub.2 is about 31.degree. C., which is lower than that of freon (for example, the critical temperature of R12 is 112.degree. C.). Thus, when the outside air temperature is high, the temperature of CO.sub.2 in the heat emitter is higher than the critical temperature. As a result, CO.sub.2 is not condensed at the outlet side of the heat emitter (segment BC does not cross the saturated liquid line).
The condition (C) of CO.sub.2 at the outlet side of the heat emitter depends on the pressure of CO.sub.2 discharged by the compressor and the temperature of CO.sub.2 at the outlet side of the heat emitter. As the outside air temperature cannot be controlled, the CO.sub.2 temperature at the outlet side of the heat emitter cannot be controlled.
Accordingly, the condition (C) can be controlled by only controlling a discharge pressure in the compressor (CO.sub.2 pressure at the outlet side of the heat emitter). That is, when the outside air temperature is high in summer or the like, the CO.sub.2 pressure at the outlet side of the heat emitter needs to be raised as denoted by E-F-G-H-E in FIG. 7, for attaining a sufficient cooling performance (enthalpy difference).
For example, the maximum CO.sub.2 pressure in the CO.sub.2 -refrigerating cycle is about ten times as high as that in the conventional refrigerating cycle where the freon is used as refrigerant.
As described above, in the CO.sub.2 -refrigerating cycle, because the maximum refrigerant pressure is much higher than that in the conventional refrigerating cycle, a heat exchanger used in the conventional refrigerating cycle cannot be applied to the CO.sub.2 -refrigerating cycle.
JP-U-63-54979 discloses a heat exchanger in which the end portion of a header tank is formed into a semi-sphere shape. The strength of the end portion of this header tank is high. However, this heat exchanger is formed by stacking plural thin plates of a predetermined shape, and by brazing them together. Thus, as this heat exchanger has many connecting portions, the pressure strength thereof is not sufficient in view of the entire heat exchanger.